1. Field of the Invention
The present invention relates to a yarn article comprising a tetrafluoroethylene polymer and a process for producing the same. More particularly, the present invention is concerned with a yarn article comprising a tetrafluoroethylene polymer, which has a specific bulk density, a specific orientation degree in an axial direction and a specific crystallinity and exhibits two specific peaks in the thermogram of differential scanning calorimetry in the course of temperature elevation. The mechanical strength, e.g., the tensile strength at break, and the tensile modulus of elasticity of the yarn article are extremely high. Therefore, the yarn article of the present invention is advantageously used as a material for producing a woven fabric, a knit, a rope and the like, and the yarn article is useful in fields where the above-mentioned properties are desired.
2. Discussion of Related Art
Polytetrafluoroethylene has excellent chemical inertness, water repellency, electrical insulating properties and the like when compared with a hydrocarbon polymer. Therefore, a yarn article comprising polytetrafluoroethylene has advantageously been used in various fields in place of a yarn article comprising a hydrocarbon polymer. However, polytetrafluoroethylene has a drawback in that because of its poor melt moldability, it was necessary to employ a special process to obtain a yarn article of the polytetrafluoroethylene.
For example, according to U.S. Pat. No. 2,772,444, a dispersion of polytetrafluoroethylene in a viscose is wet spun, and heated at a temperature of from 340.degree. to 400.degree. C. to fuse the polytetrafluoroethylene particles and, at the same time, cause the cellulose to be carbonized, followed by hot drawing, to thereby obtain a yarn article. However, this process is complicated and expensive. Further, the yarn article obtained by this process has unsatisfactory mechanical strength.
British Patent No. 813,331 and U.S. Pat. Nos. 2,776,465 and 4,064,214 disclose various modes of a process which consists in spinning an emulsion of polytetrafluoroethylene or extruding a paste of polytetrafluoroethylene, and sintering the resultant fibrous polytetrafluoroethylene at a temperature not lower than the crystalline melting point of the polytetrafluoroethylene, followed by drawing at a temperature of 340.degree. to 400.degree. C. at a draw ratio of 2 to 30 times, to thereby obtain a yarn article having a high orientation degree. However, the yarn article obtained by the above process has at the most a tensile strength of about 2 g/d and an initial modulus of elasticity of only about 20 to 60 g/d. Therefore, the yarn article obtained by the above process is insufficient in mechanical strength properties for practical application.
In the process of U.S. Pat. Nos. 3,953,566, 3,962,153 and 4,187,390, a paste obtained by mixing a lubricant, such as mineral spirit, with polytetrafluoroethylene is extrusion-molded, the resultant molded product is dried to remove the lubricant, and the dried molded product is drawn at a temperature lower than the crystalline melting point of polytetrafluoroethylene and at a high drawing rate, followed by sintering, at a temperature higher than the crystalline melting point, under a stretched condition to obtain a porous article. The porous article has high mechanical strength, even if the porous article is in the form of a yarn. However, such a porous yarn article has an apparent cross-section area larger than the cross-section area of a non-porous yarn article having the same fineness in terms of denier. With respect to the porous yarn article, the cross-section area, which contains the area of pore portions, is defined as an apparent cross-section area. The mechanical strength of the porous yarn article is not satisfactory in terms of the mechanical strength per unit apparent cross-section area because of its porous structure, as compared to the mechanical strength per unit cross-section area of a non-porous yarn article. Accordingly, the porous yarn article is not satisfactory in applications in which the use of a very fine yarn article having high mechanical strength is required. When a woven fabric is produced using the porous yarn article, since the maximum thread count per unit length or width of the woven fabric depends upon the thickness of the yarn article, the maximum thread count of the fabric made of the porous yarn article is small as compared with that of a fabric made of the non-porous yarn article having the same fineness as the porous yarn article. Accordingly, the tensile strength per unit width of the woven fabric made of the porous yarn article is lower than that of the woven fabric made of the non-porous yarn article. Therefore, when it is intended to produce a woven fabric having a high mechanical strength, it is disadvantageous to use such a porous yarn article. Moreover, the porous yarn article is generally poor in resistance to a force applied in the radial (or thickness-wise) direction, so that the porous yarn article has poor compressive resistance. For example, when a high density woven fabric made of a porous yarn article is used as a filter fabric for a prolonged period of time, the weave pattern is disarranged, due to the creep of the porous yarn article, so that the woven fabric can no longer serve as a filter fabric.
U.S. Pat. Nos. 3,953,566 and 3,962,153 also disclose a process for producing a film of polytetrafluoroethylene having a low porosity by pressing a film of polytetrafluoroethylene having a high porosity. Although the porosity of the film obtained by this process is reduced by the pressing, the film still has a porosity of about 3%, and has a structure comprised of nodes interconnected by fibrils. Further, the mechanical strength of the obtained film is not increased or rather is lowered by the pressing as compared to that of the starting film which has not yet been subjected to being pressed.
In these situations, a polytetrafluoroethylene yarn article having a very high mechanical strength and modulus of elasticity has been desired commercially.